There is this movie with Kevin Spacey who is a college professor and who sleeps with a student-"Life of David Gray" or something. First watch that movie for some good ideas and I think you will know what to do. I was asking around my school about professors and students hooking up-seems like the situation regulary occurs. Now at my school, the professor would be left go but every university is somewhat different and in fact some schools even have guidelines, you know, don't be having sex with a student who is actively taking your class but beyond that, anything goes.
I say if you can get a good grade, go for it, why not-you are in college. However, I know a lot of girls who do better their grade just because of their looks which ultimately affects some professors to be more generous to them without even putting out. I guess it's all how you approach it.